Resolve #784 https://github.com/jiaaro/pydub/issues/784

[rwmnau]

This pull request is associated with issue #784: https://github.com/jiaaro/pydub/issues/784. Either of the commits in this pull request can be taken - they're not both needed, though it won't cause a problem if they're both taken.

• Commit 1f4a152 adds type hints to the method definition (marking everything that the doc says is a string as required to be a string). There aren't type hints elsewhere in the pro
• Commit 8146622 checks if the bitrate passed into the method is an int, and if it is, it recasts it as a string before adding it to the args list. If it's not present or any other type, it leaves it alone.

[rwmnau]

This PR breaks Python 2.7 compatibility by adding type hinting to the method definition - whoops. I'll submit one with just the int check since that should be okay.